To make the diagnosis on an exhaust gas circulating device to be completed surely, by increasing frequency for establishing conditions of permitting diagnosis.

Whether or not a set first diagnosis permitting condition comes into existence in an EGR area in which a vehicle is normally run is discriminated in S1 and, when the condition does not come into existence, a second diagnosis permitting condition which is contingent on a no-load condition and a prescribed number of revolutions is discriminated (S2). When the second condition comes into existence, the circulation of an exhaust gas is forcibly started (S11). A diagnosing device selects reference values for discrimination under each condition (S2 and S10) and proceeds to the diagnoses in and after S3. In S3-S7, the device successively integrates rotation varying parameters related to the circulating rate of the exhaust gas and, after integrating the parameters a prescribed number of times, compares the computed value of the integrated parameter with the reference values for discrimination. When the computed value is smaller than the reference values, the device discriminates that a negative pressure control valve is normal and sets a diagnosis-completion flag (S8).